According to the prior art, automatic transmissions, in particular for motor vehicles, comprise planetary gearsets which are shifted by means of frictional or shifting elements such as clutches and brakes, and are usually connected with a starting element which can be operated with slip and is optionally provided with a bridging clutch, such as a hydrodynamic torque converter or a fluid coupling.
Such an automatic transmission is known, for example, from DE 199 12 480 B4 by the present applicant. It comprises three single-carrier planetary gearsets as well as three brakes and two clutches for engaging six forward gears and one reverse gear, a drive input shaft and a drive output shaft, wherein the carrier of the first planetary gearset is connected permanently to the ring gear of the second planetary gearset, the carrier of the second planetary gearset is connected permanently to the ring gear of the third planetary gearset and the drive input shaft is connected directly to the sun gear of the second planetary gearset.
Furthermore, in this known transmission it is provided that the drive input shaft can be connected by the first clutch to the sun gear of the first planetary gearset and by the second clutch to the carrier of the first planetary gearset, the sun gear of the first planetary gearset can be connected by the first brake to a housing of the transmission and the carrier of the first planetary gearset can be connected by the second brake to the housing of the transmission, whereas the sun gear of the third planetary gearset can be connected by the third brake to the housing of the transmission. The drive output shaft of the transmission is connected permanently to the carrier of the third planetary gearset and to the ring gear of the first planetary gearset.
In addition a 9-gear multi-stage transmission is known from DE 29 36 969 A1; this comprises eight shifting elements and four planetary gearsets, one planetary gearset serving as the upstream gearset and the main transmission comprising a Simpson gearset and a further planetary gearset that serves as a reversing gearset.
Other multi-stage transmissions are known, for example, from DE 10 2005 010 210 A1 and DE 10 2006 006 637 A1 by the present applicant.
In general, automatically shifted vehicle transmissions of planetary design have already been described many times in the prior art and are continually undergoing further development and improvement. These transmissions should take up little structural space, in particular requiring a small number of shifting elements, and in sequential shifting operations should avoid double shifts, i.e. an engagement or disengagement of two shifting elements at a time, so that for shifting operations in defined gear groups in each case only one shifting element is changed.
From DE 10 2008 000 428 A1 by the present applicant a multi-stage transmission of planetary design is known, which comprises a drive input and a drive output arranged in a housing. In this known transmission there are at least four planetary gearsets, denoted in what follows as the first, second, third and fourth planetary gearsets, at least eight rotating shafts—denoted in what follows as the drive input shaft, the drive output shaft and the third, fourth, fifth, sixth, seventh and eighth shafts—and at least six shifting elements including brakes and clutches, whose selective engagement produces various gear ratios between the drive input and the drive output, so that preferably nine forward gears and one reverse gear can be produced.
In this case the first and second planetary gearsets, preferably designed as minus planetary gearsets, namely ones with a negative fixed transmission gear ratio, form a shiftable upstream gearset whereas the third and fourth planetary gearsets form a main gearset.
In this known multi-stage transmission it is provided that the carriers of the first and second planetary gearsets are coupled with one another by the fourth shaft, which is connected to an element of the main gearset, the ring gear of the first planetary gearset is coupled to the sun gear of the second planetary gearset by way of the eighth shaft, which can be connected detachably to the drive input shaft by a first clutch, and the sun gear of the first planetary gearset can be coupled by means of the third shaft via a first brake to a housing of the transmission and can be detachably connected via a second clutch to the drive input shaft, whereas the ring gear of the second planetary gearset can be coupled by means of the fifth shaft via a second brake to a housing of the transmission. In addition the seventh shaft is permanently connected to at least one element of the main gearset and can be coupled by a third brake to the housing of the transmission, whereas the sixth shaft is permanently connected to at least one further element of the main gearset and can be detachably connected to the drive input shaft by means of a third clutch; the drive output shaft is permanently connected to at least one further element of the main gearset.
Preferably, in this known transmission the fourth shaft is permanently connected to the ring gear of the third planetary gearset, whereas the sixth shaft is permanently connected to the ring gear of the fourth planetary gearset and to the carrier of the third planetary gearset, and can be detachably connected by the third clutch to the drive input shaft. Furthermore, the seventh shaft is connected permanently to the sun gears of the third and fourth planetary gearsets and can be coupled by the third brake to a transmission housing. In this case the drive output takes place by way of the drive output shaft which is permanently connected to the carrier of the fourth planetary gearset. In addition, the third and fourth planetary gearsets can be combined or reduced to a Ravigneaux gearset with a common carrier and a common ring gear.
According to the prior art the shifting elements of multi-stage transmissions designed in that way, which are usually in the form of disk clutches or disk brakes, are actuated hydraulically, but this results disadvantageously in high hydraulic losses. To avoid these actuation losses, it would be particularly advantageous to use shifting elements that can be actuated only as required, for example electro-mechanically actuated shifting elements.
To enable the use of shifting elements that can be actuated as required, the shifting elements, clutches in particular, have to be easily accessible from outside.
Shifting elements that can be actuated as required are in particular understood to mean ones which consume only little energy or even none when they remain in their current shifting position. Such shifting elements may be for example shifting elements that are actuated electro-mechanically or electro-hydraulically.